1. Field of Invention
This invention relates to semiconductor memories and in particular split gate flash memory cells
2. Description of Related Art
Split gate flash memory technology requires a relatively large cell size compared to other type memory technologies. This is in part caused by misalignment problems and not being able to take advantage of self alignment techniques. Some designs of flash memory cells have multiple storage bits per each memory cell to accommodate the increased demand storage density, but this usually comes with an increased program current.
In U.S. Pat. No. 5,838,618 (Lee et al.) a method is disclosed to erase data from a flash EEPROM while electrical charges trapped in the tunneling oxide are eliminated to maintain separation of the programmed and erased thresholds. In U.S. Pat. No. 5,508,995 (Zimmer et al.) is described a split gate EPROM cell with buried bit lines on either side of a storage cell. The source for the EPROM cell is in part a buried bit line on one side of the storage cell and the drain is in part a buried bit line on the other side of the cell In U.S. Pat. No. 5,440,158 (Sung-Mu) is shown an EPROM cell with dual sidewall floating gates. Source and drain regions are formed between and on either side of the floating gates and a control gate is formed over the floating gates. In U.S. Pat. No. 5,067,108 (Jenq), an electrically conductive re-crystallized floating gate is disposed over an insulating area extending over a portion of a channel region and a drain region. A control gate partially overlaps the floating gate and extends over a portion of a source region.
With the demands for increased density for flash memory chips, it is important to create a small cell size that can be easy to shrink. The demand for increased density will require a solution to the misalignment problem in conventional split gate flash memories, and the minimizing of requirements for metalization and contact areas. To deal with the density requirement a cell architecture is required that has floating gates with source and drain areas that are in part a portion of buried bit lines and a control gate that extends beyond the cell to form in part a word line for the flash memory. Doing these items of improvement can produce an architecture for a split gate flash memory cell that will allow the cell to be reduced in size producing a higher flash memory density.